1. Field of the Invention
The present invention relates to a surface-readout type near field optical recording medium wherein recording and/or reproducing is conducted by bringing an optical head close to the recording medium surface, and in particular, a surface-readout type near field magneto-optical recording medium wherein information-recording/reproducing/erasing are conducted by utilizing laser light and a magnetic field, or a surface-readout type near field phase-change recording medium wherein information-recording/reproducing/erasing are conducted by only laser light.
2. Discussion of Background
An optical recording medium is a portable recording medium permitting large quantity and high density recording, for which there has been an increase of demand as a rewritable medium for a large volume file of computer or for recording dynamic images in current fashion of multimedia.
The optical recording medium generally comprises a transparent circular disc-like substrate made of a plastic material or the like on which a multilayer including a recording layer is formed. Recording or erasing is carried out by irradiating laser light, and reproducing is carried out by reflection light of the laser light. The optical recording medium is classified into an erasable type capable of rewriting such as a magneto-optical recording medium or a phase-change recording medium and a write-once type capable of writing only once such as CD-R. In the magneto-optical recording medium, a so-called light modulation recording has mainly been used wherein information is erased by applying a fixed magnetic field, and then, recording is carried out by applying a fixed magnetic field in an opposite direction. However, in recent years, there has been noted a magnetic field modulation system wherein a magnetic field is modulated according to a recording pattern while laser light is irradiated because such system can record correctly information in one revolution (direct over-write) at a high recording density. Demands for the phase-change recording medium have recently been increasing because it is possible to conduct the direct over-writing by an optical modulation recording system and to conduct reproducing by the using the same optical system as for CD or DVD. Further, a write-once type CD-R is widely spread because it is completely compatible with CD.
In the optical recording medium, a track or tracks are generally formed substantially concentrically or spirally, and each track is divided into several ten sectors to several hundred sectors. Each sector has a data area in which a user records his or her own data and a header area in which information for reproducing the address and so on of each sector (hereinbelow, referred to as a header information) is previously recorded. The data area has guide grooves or guide marks according to which laser light follows an intended track. Generally, the follow-up of laser light to the track is carried out by detecting a change in the intensity of reflection light of diffraction light from these guide grooves or guide marks. The header information is previously formed by preparing a concave/convex pattern in a master plate by controlling ON/OFF in the irradiation of laser light in a mastering step, the pattern being transferred to the substrate in a molding step (hereinbelow, referred to as a hard format); is formed by recording only a timing signal in the substrate, which provides datum :for recording addresses and so on, the addresses and so on being written later by the same method as the recording of data (hereinbelow, referred to as a soft format), or is formed by using these methods in combination. In any case, information recorded in the header area Ls called generically as a header information, and concave portions in the concave/convex pattern constituting the header information are called pits and convex portions are called bumps. Further, the concave/convex portion or the header information which is previously formed in the substrate is called a pre-pit portion.
Conventionally, laser for recording or reproducing is irradiated to the recording layer through the substrate. As a technique of bringing an optical head close to the recording layer to conduct recording or reproducing, a so-called near field optical recording has been noted as means for increasing recording density (Appl. Phys. Lett. 68, p. 141 (1996). In this recording method, an optical head (hereinbelow, referred to as SIL head) having a solid immersion lens (hereinbelow, referred to as SIL) is used to reduce the spot size of laser light, whereby it is possible to reproduce smaller marks which is beyond the limit of the conventional recording technique determined by a laser wavelength (xcex) of a light source (xcx9cxcex/2NA: NA represents a numerical aperture of objective lens), and recording or reproducing can be realized at a superhigh recording density. In the near field optical recording, it is necessary to bring the optical head closer to the recording medium (xcx9c100 nm or less). Accordingly, unlike the conventional optical recording medium to which laser light is irradiated to the recording layer through the substrate, a method for irradiating directly laser light to the recording layer without passing through the substrate, is used (surface-readout type recording). In this case, use of a flying type optical head having a slider is proposed in order to bring SIL head closer to the recording layer. Further, in the structure of the recording layer, the conventional optical recording medium is generally of substrate/first protective layer/recording layer/second protective layer/reflection layer. On the other hand, in the near field optical recording, there is a contrary film structure such as, for example, substrate/reflection layer/first protective layer/recording layer/second protective layer so that recording and reproducing are conducted by irradiating laser beams from the layer surface side.
In the conventional optical recording medium, the header information is recorded with the position or the length of pits having a width in a direction perpendicular to a track (which is narrower than the width of the track as in CD), and the reproducing is carried out by detecting a change in the intensity of returning light from pits due to a reflection effect when laser light is irradiated. In the case of the above-mentioned surface-readout near field optical recording, however, it was very difficult to reproduce correctly the header information by detecting the position and the length of pits having a width in a direction perpendicular to a track, which was narrower than the width of the track.
The near field optical recording medium of the present invention is constructed so that a header area in which at least an addressing signal for reproducing an address or a timing signal for providing datum for writing an address is recorded in a substrate by means of a concave/convex pattern is provided, and recording and/or reproducing is conducted by laser light using a flying optical head, wherein when a concave/convex portion which constitutes the concave/convex pattern is detected, either one of an effect caused by a distance effect wherein the reflectance is changed depending on the distance between a bottom surface of the optical head and the substrate surface and an effect caused by a diffraction effect at a step is sufficiently larger than the other, or the effect caused by the distance effect and the effect caused by the diffraction effect are not weakened each other, whereby a header information can correctly be reproduced. Further, the near field optical recording medium is constructed so that the effect by the distance effect and the effect by the diffraction effect are mutually strengthened, whereby the header information can correctly be reproduced more easily.
Namely, the present invention concerns a near field optical recording medium having a header area in which at least an addressing signal for reproducing an address or a timing signal for providing datum for writing an address is recorded by means of a concave/convex pattern formed in a substrate wherein recording and/or reproducing is conducted by laser light using a flying optical head, the near field optical recording medium being characterized in that said concave/convex pattern is constituted by concave pits or convex bumps, each width of the pits or the bumps in a direction perpendicular to a track being smaller than the beam spot, wherein |Dpxe2x88x92Df|/(Dp+Df)xe2x89xa70.05 where Dp is an intensity of returning light returned to the optical head at a position just above a pit or just above a bump and Df is an intensity of returning light returned to the optical head in a flat portion in which no pit or bump is formed, or said concave/convex pattern is constituted by concave portions and convex portions adjacent to each other wherein in a case that returning light returned to the optical head at a position just above a step portion, which is formed at the boundary of a concave portion and a convex portion which are adjacent to each other in a direction of track, is divided into a concave side and a convex side, |Dcxe2x88x92Dd|/(Dc+Dd)xe2x89xa70.05 where Dc is an intensity of returning light at a concave side and Dd is an intensity of returning light at a convex side.
The near field optical recording medium of the present invention comprises a header area in which at least an addressing signal for reproducing an address or a timing signal for providing datum for writing an address is recorded by means of a concave/convex pattern formed in a substrate wherein recording and/or reproducing is conducted by laser light using a flying optical head, the near field optical recording medium being characterized in that a change in returning light returned from the concave/convex portion formed in the header area includes at least a change due to a distance effect wherein the reflectance is changed depending on the distance between a bottom surface of the optical head and the substrate surface and a change due to a diffraction effect caused by a step, and wherein the header area is so constructed that in detecting the concave/convex portion, the effect due to the change by the distance effect and the effect by the diffraction effect are not mutually weakened each other. Further, the near field optical recording medium is constructed so that the effect due to the change by the distance effect and the effect by the diffraction effect are mutually strengthened.
The near field optical recording medium of the present invention is constructed so that the concave/convex pattern is constituted by concave pits, each width of the pits in a direction perpendicular to a track being smaller than the beam spot, wherein when Ht is the distance between a bottom surface of the optical head and a flat portion in which no pit is formed, and Ha is the distance between the bottom surface of the optical head and a bottom surface of a pit, the reflectance R has a negative inclination in a range of from Ht to Ha.
Further, the near field optical recording medium of the present invention is constructed so that the concave/convex pattern is constituted by convex bumps, each width of the bumps in a direction perpendicular to a track being smaller than the beam spot, wherein when Ht is the distance between a bottom surface of the optical head and a flat portion in which no bump is formed, and Hb is the distance between the bottom surface of the optical head and an upper surface of a bump, the reflectance R has a positive inclination in a range of from Hb to Ht.
Further, the near field optical recording medium of the present invention is constructed so that the concave/convex pattern is constituted by concave portions and convex portions which are adjacent to each other, wherein when Hc is the distance between a bottom surface of the optical head and a bottom surface of a concave portion, and Hd is the distance between the bottom surface of the optical head and an upper surface of a convex portion, the reflectance R has a positive inclination in a range of from Hc to Hd. In this case, when concave portions or convex portions in the concave/convex pattern formed in the header area are at an equivalent position in adjacent tracks, it is preferable that the concave portions or the convex portions are connected in a direction perpendicular to the tracks by bridging the adjacent tracks. Further, when concave portions or convex portions are not at an equivalent position in adjacent tracks, it is preferable that a step formed at the boundary of a concave portion and a convex portion, which are adjacent to each other in adjacent tracks, is shifted to a side of the track in which the convex portion is formed, with respect to the boundary of the adjacent tracks.